Field of the Invention
The present application relates generally to detection of interference due to passive intermodulation (PIM) products of transmitted signals in a wireless network or networks, and more specifically, but not exclusively, to generating a diagnosis from PIM detection results for a plurality of cell sites, the diagnosis comprising a likely cause of PIM in the network and/or predictions of a likelihood of experiencing PIM in the network or networks.
Description of the Related Technology
Passive intermodulation products may be generated in a wireless network when two or more signals are transmitted at different frequencies along a signal path including a passive (unpowered) device having a non-linear transmission characteristic; these products differ in frequency from the signals from which they were generated, and may potentially cause interference to other signals. The generation of passive intermodulation products is becoming a problem of increasing importance in modern wireless communication systems, and in particular cellular wireless systems, since the radio frequency spectrum available has been steadily expanded as additional bands have become available, and the pattern of allocation of uplink and downlink bands within the available spectrum for use by various cellular systems, such systems using GERAN (GSM EDGE Radio Access Network), UTRAN (UMTS Terrestrial Radio Access Network) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) radio access networks, and by various operators, is complex and territorially dependent. In this environment, and in particular at a cellular radio base station at a cell site, it is likely that circumstances arise in which passive intermodulation products generated from transmitted carriers in one or more downlink bands would fall within an uplink band in which signals are received at the base station. Intermodulation generated by non-linear characteristics of active components such as power amplifiers may generally be dealt with at a design stage by appropriate frequency management and filtering, but intermodulation caused by non-linear characteristics of passive components, so called passive intermodulation (PIM), may prove more difficult to manage. Many passive components may exhibit a non-linear transmission characteristic to some degree, for example due to an oxide layer at a metal to metal contact, and the non-linear characteristic may develop with time as an ageing process of the component. Furthermore, PIM may be caused by components in the signal path between the transmitter and the receiver that are external to the active transceiver equipment and which may be outside the operator's control, such as metallic objects on the antenna tower or other objects in the signal path through the propagation environment, such as fences; this is known as the “rusty bolt” effect. PIM may also be caused in passive components, such as antennas or diplexers, which are external to the active transmitter and receiver equipment.
Interference due to PIM received at an incoming signal receiver, typically at a base station such as an eNodeB located at a cell site, may be detected by a PIM detection device, as described in UK patent application GB1208921.5. However, if a PIM detection device detects PIM, it is difficult to diagnose likely causes of PIM at a cell site at which it is located. It is also difficult to diagnose the likelihood of PIM becoming a problem at a given cell site in the future. Furthermore, it is difficult for a wireless network operator to diagnose PIM problems across a wireless network such as a Public Land Mobile Network (PLMN) that may comprise many hundreds of cell sites covering a large geographical area, and which may comprise a variety of equipment types, subject to varying environmental conditions. In addition, it is difficult to diagnose PIM problems caused by interactions between carriers of two or more PLMNs.